A sonar system is limited in detection ability either by reverberation or by noise. A target having a very low doppler frequency shift because it is slow moving may be reverberation-limited because the return frequency is so close to the transmitted frequency. A fast moving target on the other hand may be at longer range and while there is adequate frequency shift to enable receiver operation to separate the echo from reverberation, ambient noise becomes a problem.
Because both these limitations may apply at the same time with different targets, it has been attempted to overcome this problem by operating the system in different modes sequentially. For example, an FM pulse is transmitted to detect targets otherwise limited by reverberation while a continuous wave pulse is used in the next cycle to detect noise-limited targets. As the transmitter shifts from one mode to the other on alternate cycles the receiver is similarly switched.
However, in sonar, detection usually depends on the spatial correlation of several echoes in order to differentiate between false alarms and targets. Operation in this sequential alternate mode results in greater time being required to receive the necessary number of responses to detect and differentiate between targets.
Many sonar systems nowadays operate using digital technology because of the ability to combine many channels into a single channel by means of time-division multiplexing. However, this gives rise to the situations where the failure of a single component can cause the loss of the entire system. To avoid this risk, it is known to use a redundant channel which is available to replace the primary channel in the event of failure.